1. Field of the Invention
This invention relates to a projection exposure apparatus provided with a projection optical system, and more particularly to an exposure apparatus provided with a projection optical system used to project a minute pattern formed on a photomask or a reticle onto a semiconductive wafer and having the optical characteristic thereof controlled very strictly.
2. Description of the Prior Art
In a projection exposure apparatus for producing semiconductor devices such as super-LSI by a photo-lithograph including the step of projecting the image of the pattern on the photomask or the reticle onto the semiconductor wafer having photoresist applied thereto, at a predetermined magnification, by the use of a projection lens, matching accuracy is mentioned as one of the important performances thereof. What is important of the factors which affect the matching accuracy is the magnification error of the projection optical system. The size of the pattern used in the super-LSI has had a the strong tendency toward minuteness year by year and along therewith, the need for improved matching accuracy has become stronger. Accordingly, the necessity of keeping the projection magnification at a predetermined value has become very high. At present, the error of the magnification of the projection optical system is made negligible by adjusting it during the installation of the apparatus. However, the projection optical system of the exposure apparatus, for example, the reduction projection lens, absorbs part of illuminating energy and the temperature thereof rises. Therefore, if the illuminating light continues to be applied to the projection lens for a long time or if the exposure operation is continuously effected for a long time, the magnification may vary to an a significant degree. Not only the magnification may fluctuate, but also the position of the imaging plane of the projection lens may fluctuate in the direction of the optic axis. Therefore, if semiconductor devices such as super-LSI are manufactured by the use of an exposure apparatus provided with such a projection optical system, the matching accuracy will be reduced and unsatisfactory resolution by the fluctuation of the imaging plane may result, and this has led to the disadvantage that the yield is substantially reduced.